Technique for charging dielectric surfaces to high voltage

ABSTRACT

A technique for charging a stationary dielectric surface by corona discharge is described in which the voltage applied to a stationary corona wire is steadily increased over a time period to cause the gradual increase in voltage level on the dielectric surface over the same time period. The dielectric surface voltage increases with the wire voltage. The potential difference between wire and plate is kept below the sparking or arcing voltage and above the threshold voltage. In an arrangement employing a moving dielectric surface, plural corona wires are maintained at successively higher voltages to increase the charge on the dielectric in steps, each step being such as to maintain the potential difference between wire and surface below the arcing voltage.

United States Patent Gallo, Jr. et al.

[ TECHNIQUE FOR CHARGING DIELECTRIC SURFACES TO HIGH VOLTAGE lnventors: Charles F. Gallo, Jr.; Thomas J.

Hammond, both of Penfield, NJ.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Dec. 5, 1973 Appl. N0.: 422,050

US. Cl 317/262 A, 250/325, 250/326 Int. Cl. H0lt 19/00 Field of Search 317/262 A, 262 R, 3, 2 R;

250/324-326; 55/103, 150, DIG. l

[56] References Cited UNITED STATES PATENTS 3/1943 Hewitt 317/2 R 7/1972 Matsumoto et al 317/262 A Mar. 25, 1975 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney, Agent, or FirmRobert J. Bird [57] ABSTRACT A technique for charging a stationary dielectric surface by corona discharge is described in which the voltage applied to a stationary corona wire is steadily increased over a time period to cause the gradual increase in voltage level on the dielectric surface over the same time period. The dielectric surface voltage increases with the wire voltage. The potential difference between wire and plate is kept below the sparking or arcing voltage and above the threshold voltage. In an arrangement employing a moving dielectric surface, plural corona wires are maintained at successively higher voltages to increase the charge on the dielectric in steps, each step being such as to maintain 3 Claims, 4 Drawing Figures ALWAYS v wz I ALWAYS v TIME (DRUM ANGLE OR POSITION) PATENTEDHAR251975 3,873,895

PRIOR A T T ALWAYS V F16 s C v w ALWAYS v P 0 VO TIME i ALWAYS v v ALWAYS v FIG. 2

TIME

FIG. 3

wa v j ALWAYS v w2 t P) ALWAYS v NS w1 FIG. 4

TIME (DRUM ANGLE OR POSITION) TECHNIQUE FOR CHARGING DIELECTRIC SURFACES TO HIGH VOLTAGE BACKGROUND OF THE INVENTION This invention relates generally to the field of electrostatics and particularly-to a technique for charging dielectric surfaces to high voltage levels.

One application for charging devices is in xerography in which a photoconductive dielectric surface is charged with positive or negative charges, then exposed whereby the selective discharging of the photoconductor leaves a latent electrostatic image of the original object on the surface.

One present technique for applying electrostatic charge to a dielectric surface is by means of a corona discharge. In such an arrangement, a corona generating device is placed near the dielectric surface to be charged and ions generated around the corona wire flow to the dielectric surface because of potential difference between the wire and the surface. Further details regarding present corona charging devices and their relation to xerography are available in Xerography and Related Processes by Dessauer and Clark, published in 1965 by The Focal Press. The relevant disclosures in this book are incorporated in this specification by reference.

In corona charging devices typical of the prior art (see FIG. 1), a fixed voltage level is applied to a corona wire and charge is deposited and accumulated on the nearby dielectric surface or plate. The wire voltage must be high enough that the electric field surrounding the wire ionizes the air in the immediate vicinity. This minimum voltage, which is required to initiate a corona current, is called the threshold voltage (V Another factor is that the corona current is dependent upon the voltage difference or potential difference between wire and plate. Thus, if a corona wire is used to charge a dielectric plate, the current is determined by the voltage difference between the wire and the plate. If a voltage V is applied to the wire, an initial current I,,. is drawn. This current will charge the dielectric plate and thereby reduce the potential difference between the wire and the plate. The corona current then decreases along a current-voltage curve until the voltage difference between the wire and the plate surface is reduced to the threshold voltage V as a limit. In other words, the corona current asymptotically approaches zero.

The voltage on the plate, V,,, increases with time during the charging and asymptotically approaches the value of the wire voltage less the threshold voltage. That is, the limit of V V V For wire-to-plane coronas, the maximum voltage which corona is finally capable of applying to dielectric plates (V max) is determined by the maximum voltage which can be applied to the wire, which is essentially the sparking or arching voltage V Thus, by extending the foregoing equation to its limit, V max V V By means of the present invention, the voltage limits inherent in such prior art charging methods are removed.

DRAWING FIG. I is a curve of wire and plate voltages plotted against time in a typical prior art charging process.

FIG. 2 is a curve similar to that shown in FIG. 1 showing the process as practiced in the present invention.

FIG. 3 is a schematic diagram of a xerographic drum and corona charging system.

FIG. 4 is a curve of wire and drum voltages plotted against time in a charging process practiced on the system represented in FIG. 3.

DESCRIPTION Referring now to FIG. 1, a typical prior art charging process is represent including a constant wire voltage and an increasing plate voltage asymptotically approaching the value of V,,. V, as a limit. Two conditions are observed. First, the potential difference between wire and plate must be less than the arcing voltage V to avoid break-down. Second, the potential difference between wire and plate must be greater than the threshold voltage V to maintain current flow. FIG. 1 represents graphically how these conditions are met. The maximum plate voltage obtainable with this conventional scheme is V, V

Referring now to FIG. 2, a charging process according to this invention is represented in which the wire voltage V,,. is increased steadily during the charging process and concurrently with this increase, the plate voltage V similarly increases. The necessary conditions stated above are met. That is, at all times the potential difference between wire and plate is less than the arcing voltage but greater than the threshold voltage to maintain current flow. Simply by staying within these limits, the wire voltage can be increased to bring the plate voltage to any desired level, subject only to other limitations, such as breakdown of the dielectric plate, which are not material to this invention. This scheme circumvents the usual limitation imposed by the sparking voltage. It should be noted that this process is primarily relavant to stationary wire(s) over a stationary dielectric substrate. Referring now to FIG. 3, the charging concept described in connection with FIG. 2 is applied, somewhat modified, to a xerographic rotating drum apparatus. In this schematic, a xerographic drum 2 is rotated relative to an exposure station 4 and at some point thereafter, the drum 2 is in a discharged or uncharged condition. Prior to the next exposure, the drum passes relative to three corona generating wires W W W having successively higher voltages applied thereto. That is, corona wire W, is at a first voltage level, W a second voltage level, and W a third voltage level. This charging process of drum 2 is shown graphically in FIG. 4 in a form similar to that of FIG. 2. During the time when a given increment of drum area is adjacent to corona wire W it charges to a certain voltage level. The drum area then passes under corona wire W which further increases the drum voltage, and finally corona wire W which increases the drum voltage to its final, unusually high value. The result is a charge level higher than would be attainable by a single corona discharge device which would be limited by the sparking voltage.

The foregoing description of the method of this invention is given by way of illustration and not of limitation, the curves and the schematic are representative only. The concept and scope of the invention are limited only by the following claims and equivalents thereof which may occur to others skilled in the art.

What is claimed is:

l. A method of applying electrostatic charge to a stationary dielectric surface by means of a stationary co- 3,873,895 3 4 rona electrode operatively disposed relative to said disurface, electric surface, said method including the steps of; d. the successively higher voltages applied to said coa. applying a voltage to said corona electrode at a rona electrodes being such that the potential differlevel relative to that of said dielectric surface above ence between said electrodes and said dielectric the threshold level required to ionize the surround- 5 surface as it passes said electrodes remains being atmosphere and generate corona current, and tween said breakdown level and said threshold below the breakdown level at which arcing occurs, level, so that corona current flows to said dielectric surwhereby the voltages applied to said corona elecface, trodes and said dielectric surface may be increased b. increasing the voltage on said corona electrode in above, said breakdown level.

relation to the increase of voltage on said dielectric 3. A method of applying electrostatic charge to a disurface according to the condition that the potenelectric surface by means of a corona electrode operatial difference between said corona electrode and tively disposed relative to said dielectric surface, insaid dielectric surface remains between said breakcluding the steps of: t down level and said threshold level, while the volta. applying a voltage (V,,.) to said corona electrode age of said corona electrode is increased above the at a level such that the difference in potential bebreakdown level, tween said corona electrode and said dielectric surwhereby the voltage of said dielectric surface may be face is above the threshold level (V required to increased above the breakdown level. ionize the surrounding atmosphere and generate 2. A method of applying electrostatic charge to a corona current, and below the breakdown level moving dielectric surface by means of a corona dis- (V,) at which arcing occurs, so that corona current charge, including the steps of: flows to said dielectric surface to accumulate eleca. applying successively higher voltages to a plurality trostatic charge thereon,

of corona electrodes operatively disposed relative b. increasing the voltage (V of said corona electo said dielectric surface, trode with reference to the accumulated voltage b. moving said dielectric surface relative to said co- (V of said dielectric surface so that the difference age,

c. the lowest voltage on said electrodes being at a level above the threshold level required to ionize the surrounding atmosphere and generate corona, and below the breakdown level at which arcing ocin potential between said corona electrode and said dielectric surface (V V,,) is kept above the threshold level (V,,) and below the breakdown level (V,) while the voltage (V,,.) of said corona electrode is increased above the breakdown level (V,,) and whereby the voltage (V,,) of said dielectric surface may be increased above the breakdown level (V curs, so that corona current flows to said dielectric 

1. A method of applying electrostatic charge to a stationary dielectric surface by means of a stationary corona electrode operatively disposed relative to said dielectric surface, said method including the steps of; a. applying a voltage to said corona electrode at a level relative to that of said dielectric surface above the threshold level required to ionize the surrounding atmosphere and generate corona current, and below the breakdown level at which arcing occurs, so that corona current flows to said dielectric surface, b. increasing the voltage on said corona electrode in relation to the increase of voltage on said dielectric surface according to the condition that the potential difference between said corona electrode and said dielectric surface remains between said breakdown level and said threshold level, while the voltage of said corona electrode is increased above the breakdown level, whereby the voltage of said dielectric surface may be increased above the breakdown level.
 2. A method of applying electrostatic charge to a moving dielectric surface by means of a corona discharge, including the steps of: a. applying successively higher voltages to a plurality of corona electrodes operatively disposed relative to said dielectric surface, b. moving said dielectric surface relative to said corona electrodes so that a given area of said surface passes relative to said electrodes in series from the electrode of lowest to the electrode of highest voltage, c. the lowest voltage on said electrodes being at a level above the threshold level required to ionize the surrounding atmosphere and generate corona, and below the breakdown level at which arcing occurs, so that corona current flows to said dielectric surface, d. the successively higher voltages applied to said corona electrodes being such that the potential difference between said electrodes and said dielectric surface as it passes said electrodes remains between said breakdown level and said threshold level, whereby the voltages applied to said corona electrodes and said dielectric surface may be increased above said breakdown level.
 3. A method of applying electrostatic charge to a dielectric surface by means of a corona electrode operatively disposed relative to said dielectric surface, including the steps of: a. applying a voltage (Vw) to said corona electrode at a level such that the difference in potential between said corona electrode and said dielectric surface is above the threshold level (Vo) required to ionize the surrounding atmosphere and generate corona current, and below the breakdown level (Vs) at which arcing occurs, so that corona current flows to said dielectric surface to accumulate electrostatic charge thereon, b. increasing the voltage (Vw) of said corona electrode with reference to the accumulated voltage (Vp) of said dielectric surface so that the difference in potential between said corona electrode and said dielectric surface (Vw - Vp) is kept above the threshold level (Vo) and below the breakdown level (Vs) while the voltage (Vw) of said corona electrode is increased above the breakdown level (Vs) and whereby the voltage (Vp) of said dielectric surface may be increased above the breakdown level (Vs). 